1. Field of the Invention
The present invention relates to a machine tool and particularly to a machine tool in which programs relating to peripheral devices fixed to the machine tool can easily be customized afterward by programmable sequence control with safety function (safety sequence ladder) embedded in the machine tool without addition of safety devices, safety sequencers, or the like made of hardware circuits such as safety switches and safety relays in case where safety is desired to be ensured not only for a main body of the machine tool but also for a state including the peripheral devices.
2. Description of the Related Art
In Japanese Patent Application Laid-Open No. 2005-227873, for instance, a programmable sequence controller has conventionally been known that includes embedded safety circuits configured with sequence programs which cannot be modified by users, for minimum safety protection of an operator from a machine tool, and user safety circuits which can freely be modified by the users and that has a built-in safety function which ensures the minimum safety of the operator operating a machine even if the user safety circuits are inappropriate.
In case where safety is desired to be ensured for a state of a machine tool including peripheral devices, conventionally, the peripheral devices can be controlled with addition of safety switches, safety relays, and/or the like and with configuration of safety circuits by hardware, or safety software can be prepared and added by means different from software for a main body with addition of safety sequencers.
There is a technique for ensuring safety in a machine using a plurality of CPUs in which cross-check among program areas that are used respectively by the CPUs is carried out. In a machine in which a plurality of CPUs are used, as illustrated in FIG. 5, for instance, program areas are classified into fixed program areas (sections to be used by a main body of the machine) controlled by a CPU 1 and a variable program area for which programs can freely be prepared by manufacturers of peripheral devices, users, or the like. Programs controlled by a CPU 2 are configured in a fixed program area. A safety function area is included in the fixed program area. For programs of the safety function area controlled by the CPU 2 and programs of a safety area in the CPU 1, cross-check in the fixed program area is carried out so that safety is ensured.
A prior art technique disclosed in Japanese Patent Application Laid-Open No. 2005-227873 described above, however, is a technique for constructing the programmable sequence controller with safety function that enables safety function at an initial design stage for control logic of the machine and has a problem in that it is difficult for manufacturers of peripheral devices to freely arrange and add the control logic of the safety function after completion of the machine.
In the prior art technique in which the safety switches, the safety relays, and/or the safety sequencers are added, the peripheral devices are controlled with the configuration of the safety circuits with the addition of the safety switches, the safety relays, and/or the safety sequencers in case where safety of an operator is required to be ensured for the peripheral devices. The addition of components for securing safety, however, entails much costs and configuration by software may fail to reliably ensure structural safety.
Among safety functions for ensuring safety in a machine using a plurality of CPUs, there are a function of monitoring operations at s short cycle and a function of monitoring safety signal status. Processing in the CPU 2 imposes a heavy burden on the CPU because mutual monitoring processing is carried out at the same cycle for all processes. The CPU 1, which is a CPU dedicated to a programmable machine controller (PMC), has sufficient processing ability, while the CPU 2, which carries out processing in spare time of the CPU for CNC, has insufficient processing ability so that it is difficult to prepare large-volume software therefor.
In case where programs for peripheral devices are added to the variable program area, such a configuration is insufficient as safety circuit because input-output signals for the peripheral devices have a single structure. Compensation for this requires additional hardware circuits, which entail much effort and great cost.
Furthermore, throughput of control software for a numerical controller has been increasing year by year due to attainment of demands for multi-axis, multi-path, and high-functionality features of machine tools and, in particular, increase in throughput for axis control processing due to the multi-axis features has been remarkable. In addition, it has been demanded that default cycle of the axis control processing with nearly real-time characteristic be further shortened for attaining machining with higher speed and higher accuracy and a problem is thereby caused in that greater amount of processing is required to be carried out in shorter period of time as a result.